The invention relates to identifying a call in a telecommunications system, and particularly to identifying a call in a system predetermining the length of an identifier identifying the call on an interface.
The latest mobile communication systems are designed to provide open interfaces between the different parts of the system. Such interfaces may include air interfaces and interfaces between two different networks. In connection with an open interface, functions taking place via the interface are clearly determined. For example, the system specifications define how a call is identified such that each subscriber receives calls that are only intended for him or her and the network knows which messages are associated with a certain call. In mobile communication systems, for example, a single message is identified as being associated with a certain call by means of the value of a call identifier included in the message. A mobile station only chooses those messages from the air interface that have the same call identifier value as the call identifier value the mobile station received in connection with call setup.
The problem with the arrangement described above is that the number space defined in the specifications for the call identifier used on open interfaces does not necessarily suffice to cover all calls occurring in the entire network area so as to provide each call with a unique, unambiguous identifier. For example the TETRA (Terrestrial Trunked Radio) standard established by ETSI (European Telecommunications Standards Institute) defines the length of the call identifier used on interfaces as 14 bits. If a network in accordance with the TETRA standard comprises 200 exchanges and the calls are evenly spread out over the whole network, each exchange has thus only about 80 identifiers available. Such a low number of identifiers is insufficient for an exchange that processes hundreds or thousands of calls simultaneously.
A solution to the above problem is to divide the network into smaller ones and use a unique network identifier for each network. Since the call identifier is network-specific, a call can be identified globally by means of the network identifier. Dividing the network into smaller ones by different network identifiers directly affects the users, however, because it is necessary to use network identifiers in connection with subscriber telephone numbers. Hence, despite the operator""s preference for one large network only it in practice has a plurality of small networks.
An object of the invention is thus to provide a method and an apparatus implementing the method so as to solve the above problems. The objects of the invention can be achieved with a method for identifying a call in a telecommunications system comprising at least one network and terminals, the system determining at least one interface for the network, the interface being connected with the terminals or other networks via the interface, and a first identifier that is used on the network interfaces for identifying the call in call messages associated with the call, the method being characterized by comprising the following steps: dividing the network into smaller sub-networks such that the first identifier used on the interface is unique in the sub-network area, determining a second identifier to be used for identifying the call in the whole network area, allocating one value of the second identifier to the call in connection with call setup, allocating one or more first values of the first identifier to the call in connection with call setup, binding each first identifier value to the value of the second identifier, replacing in call messages the value of the second identifier with the value of the first identifier before transmitting the message from the network via the interface, and replacing in the call messages received via the interface the value of the first identifier with the value of the second identifier in the network.
The concept xe2x80x98first identifierxe2x80x99 herein refers to an identifier defined in the system standards which is used on the network interfaces for identifying a call in call messages associated with the call.
The concept xe2x80x98call messagexe2x80x99 herein refers to data transmitted via the interfaces and associated with the call. Within the scope of the present application, the concept xe2x80x98dataxe2x80x99 should be understood in a wide sense to refer to any information transmitted in a digital telecommunications system. Such information may include signaling information, speech encoded into a digital form, data traffic between computers, telefax data, short program-code pieces and video image, etc.
The invention further relates to a system to which the method of the invention can be applied, the system comprising at least one network for receiving and transmitting call messages associated with a call, the network comprising first allocation means for allocating the first value of the first identifier in connection with call setup, the first identifier being used for identifying the call on an interface and the number space of the first identifier being predetermined, and transmission means for transmitting the first identifier to a terminal in connection with call setup, and terminals connected to the network via the interface and receiving in connection with call setup the first value of the first identifier from the network and transmitting to the network call messages associated with the call and equipped with the first identifier value and receiving from the network call messages, the first identifier value included in the messages being the same as the first identifier value in the terminal, the system being characterized by the first allocation means being arranged to allocate the first value of the first identifier to each terminal or group of terminals engaged in the call, the first values possibly differing from each other, and the network further comprising at least two sub-networks, the number space of the first identifier being sufficiently large in the area of the sub-networks to identify the calls on the interface, second allocation means for allocating a second identifier value from a number space determined for a second identifier in connection with call setup for identifying the call in the whole network, call control means for controlling the call and for collecting information about the call on the basis of the second identifier value, memory means for binding each first value to the second identifier value, replacement means for replacing the second identifier value in the call message with the first identifier value when the message is transmitted from the network to the terminal, and for replacing the first identifier value with the second identifier value in the call message received from the terminal.
The invention further relates to a network node which can be utilized in the method and system of the invention, the network node comprising at least one input channel and one output channel, reception means for receiving call messages associated with a call, and transmission means for transmitting the call messages associated with the call towards a target address, the network node being characterized by being arranged, in response to a call setup request received from a terminal, to allocate a first identifier value to each terminal or group of terminals engaged in the call in the network node area to be used for identifying the call on an interface between the network and the terminal, to allocate a second identifier value to the call for identifying the call in the network, to bind each first identifier value to the second identifier value, and to initiate call control using the second identifier value.
The invention is based on the idea of bringing into use, in addition to the call identifier used on an open interface, also an internal call identifier used in the network which is sufficiently long to identify all simultaneously ongoing calls in the network, and dividing the network into sub-networks having call identifiers that are unique inside the sub-networks. The internal call identifier is bound to the call identifier. Hence, two different identifiersxe2x80x94one internal identifier and one or more call identifiersxe2x80x94are associated with one call. The most significant advantage of the invention is that the length of the call identifier does no longer restricts the size of a network, since ail on-going calls in the network can be identified by means of the internal call identifier. The length of the internal identifier can be freely determined by the operator when the network is being built. In addition, the operator can readily change the sub-network division according to network load. A further advantage of the invention is that the invention can be applied to existing networks only by changing the network control software. Open interface messages can remain unchanged.
In a preferred embodiment of the invention a new first identifier value is allocated to a terminal in connection with call setup. This has the advantage that a mobile station can freely move in the area of different sub-networks and the call can be identified regardless of the sub-network being changed.
In another preferred embodiment of the invention the first identifier values are allocated to the different sub-networks such that the values of adjacent sub-networks differ from each other. This is to ensure that the terminal does not receive call messages intended for another terminal in connection with handover before receiving a new first identifier value. It is further ensured that messages transmitted by the terminal do not end up in the call control processes of another call in the system; this is possible if the same first identifier value is used in adjacent sub-networks.
Preferred embodiments of the method, system and network node of the invention are disclosed in the attached dependent claims 2 to 7, 9 to 12 and 14.